1. Field of the Invention
The invention is related to the field of solid-state memories and, in particular, to memory arrays having memory cells formed from metallic material, such as a metallic alloy that includes Fe and Rh.
2. Statement of the Problem
Magnetic tunnel junction (MTJ) devices and other magnetic devices have been proposed as memory cells for use in cross-point memory arrays, such as a nonvolatile magnetic random access memory (MRAM) array. An MTJ device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ device a function of the relative orientations and spin polarizations of the two ferromagnetic layers. Usually the tunneling probability of the charge carriers is highest when the magnetic moments of the ferromagnetic layers are parallel to one another. Thus, in MRAM the electrical resistance of an MTJ memory cell is in its lowest state when the magnetic moments or magnetizations of both ferromagnetic layers are parallel, and is in its highest state when the magnetic moments are antiparallel.
The electrical leads of the MRAM array are referred to as the bit line and the word line. For writing to a cell, current is passed down the bit and word lines. The sum of the magnetic fields that are generated by the current flowing down both lines is able to switch the magnetization of the free ferromagnetic layer in that cell. During the read process, sense current passes from the word line through the cell into the bit line. Depending on the relative orientation of the free and fixed ferromagnetic layers, the cell being read is in either a high or a low resistance state.
Other cross-point memory arrays use a mix of metallic layers and semiconductor layers to form the memory cells of the memory arrays. One problem with present MTJ MRAMs and other cross-point memory arrays using semiconductor materials in the memory cell is the generally high resistance-area product leading to increased resistance of the memory cell as its size is scaled down. Therefore the resistance of the present memory cells may be higher than desired. The high resistance reduces the speed of writing to the memory cells, which negatively affects the overall data rate of the memory array.
Moreover MTJ MRAM and similar cells are comprised of a plurality of metallic and insulating or semiconducting layers, making their fabrication and operation complex processes. For example in an MRAM cell it is required that the magnetic field from the writing current only reverses the free layer magnetization. Thus the fixed layer magnetization needs to be sufficiently pinned. This is typically achieved by exchange coupling the fixed layer to an antiferromagnet. Thus both the electrical and magnetic properties of every layer need to be accounted for.